A haptic engine (also referred to as a vibration module) is a linear resonant actuator that determines one of acceleration, velocity and displacement of a moving mass. Either one of electrical sensing or magnetic sensing can be conventionally used for measuring displacements of the mass moving in the haptic engine. An example of electrical sensing, that is referred to as back electromotive force (bEMF) sensing, is based on sensing current-voltage of a coil that interacts with a magnet attached to the moving mass. For certain applications, accuracy of an absolute value of displacement measured by bEMF sensing may be insufficient because the coil's resistance changes with temperature. An example of magnetic sensing, that is referred to as Hall sensing, is based on sensing Hall voltages using Hall sensors that interact with a magnet attached to the moving mass. A displacement measuring system based on conventional Hall sensing can be expensive to calibrate as the conventional Hall sensing uses a look-up-table calibration to linearize displacement sensitivity. Additionally, conventional Hall sensing can have displacement sensitivity dead-zones when a Z-offset between the Hall sensors and the magnet exceeds a small Z-offset threshold. Conventional Hall sensing is also susceptible to interference from external magnetic fields.
Some of the above issues are remedied if measuring displacement of a mass moving in a haptic engine is performed using optical sensing. For instance, a conventional optical system can be used in conjunction with conditioning electronics, for measuring displacements of a mass in a vibration module. Such a conventional optical system includes a light emitting diode (LED) module, a striped optical pattern attached to the moving mass, and a photodetector array module. In such a vibration module, position of the striped optical pattern can be determined relative to a beam provided by an LED module. The photodetector array module is used to image the striped optical pattern illuminated by the beam. Each of (i) a bias used to power the LED module and (ii) an output signal of the photodetector array module is conditioned by a signal processing module that is configured based on a conventional transceiver architecture. As part of the conventional transceiver architecture, the signal processing module operates essentially in class-A mode and, hence, it includes numerous analog circuits, e.g., op-amps, digital POTs, and trans-inductance amplifiers (TIAs). As such, the signal processing module, operated in such a conventional transceiver architecture, is power hungry, and hence it can be costly to operate. Further note that, because a photodetector array is used to image the striped optical pattern illuminated by a single LED, the size of the vibration module in the Z-direction tends to grow unnecessarily in order to facilitate optical focusing between the LED, the striped optical pattern, and the photodetector array.